3rd generation partnership project (3GPP) long term evolution (LTE) is an improved version of a universal mobile telecommunication system (UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPP LTE.
A micro cell, a femto cell, and a pico cell, etc., having a small service area can be installed in a specific location of a macro cell having a wide coverage. Such a network environment is called a heterogeneous network.
In a heterogeneous network environment, if the number of user equipments (UEs) is increased, traffic of the macro cell is overloaded, and thus service quality or service durability of the UE may deteriorate. To address this problem, some of the UEs which receive a service from the macro cell can be allowed to receive a service from another cell having a small coverage within a macro cell coverage. This is called offloading.
In order to offload traffic of the macro cell to another cell, when the UE is located in a coverage of a cell having a small coverage, the UE must be able to preferentially consider the cell as a mobility target cell. For this, the UE applies an offset for the cell when performing mobility evaluation.
For offloading, when it is implemented to apply an offset for a frequency of a cell having a small coverage irrespective of a location of the UE or a service environment of the UE, the offset can be applied to another macro cell having the same frequency, and thus the macro cell can be selected as a mobility target. This may cause a result opposite to the original purpose of offloading which distributes traffic from the macro cell to the cell having the small coverage.
In addition, the UE may be unable to receive a normal service from a serving cell to measure a cell which is not located in a coverage of another cell or which is not suitable for moving. This causes a problem in that service quality of the UE deteriorates since an unnecessary measurement procedure is performed.
Accordingly, there is a need for a method capable of selectively performing mobility evaluation by the UE on the basis of an offset by providing a domain condition for applying the offset when the UE performs mobility evaluation.